In a prior art threshold detector (sometimes referred to as a "zero-crossing" detector), the output of a sensor is processed by circuitry such as that shown in FIG. 1.
In the illustrated arrangement, a sensor 10 (such as a reluctance sensor) develops an output signal A which, as shown by waveform A in FIG. 2, is a sinusoidal type signal superimposed on a threshold level V. At time t.sub.1, the signal A crosses the threshold V, thereby generating a "threshold-crossing". In the case where the threshold level V is zero volts, the transition at t.sub.1 is referred to as a "zero-crossing". The purpose of the circuity shown in FIG. 1 is to develop a binary output signal that has a single transition (as opposed to multiple, unwanted transitions) at t.sub.1, and that is substantially free of any noise that may be superimposed on the signal A.
Referring again to FIG. 1, the signal A is coupled to the input of a zero-crossing (or threshold-crossing) detector 12 that generates a binary level output signal B (see waveform B in FIG. 2) having a positive-going transition that occurs at time t.sub.1.
The sensor signal A is also applied to an integrator 14 which applies an integrated version of the signal A to a threshold detector 16. The output of the detector 16 is a binary signal C (see waveform C in FIG. 2). This signal C is applied to the clock (C) input of a flip-flop 18, while the signal B is applied to the reset (R) input of the same flip-flop.
The purpose of the flip-flop is to generate a noise-free output signal D (see waveform D in FIG. 2) that has an "arm" transition and a "fire" transition. The "arm" transition is included for the purpose of establishing an amplitude level from which one can generate the "fire" transition. The "fire" transition is the important one, as it represents the time when the sensor signal experiences its threshold-crossing. In a typical automotive application, the "fire" transition gets counted, or otherwise used, to form a timing reference for a fuel injector or the like.
A problem with the foregoing approach is that, in some applications, extra circuitry may be needed to ensure that the integrated sensor signal (i.e., the signal formed by the integrator 14 and the threshold detector 16) has a fast enough rise time and/or fall time to reliably clock the arming of the output signal. In FIG. 1, for example, the signal applied to the "clock" input of the flip-flop 18 must have a relatively rapid transition in order to reliably clock the flip-flop and thereby generate the "arm" transition shown in waveform D. While in many applications the flip-flop can be reliably clocked if the integrated sensor signal is properly processed (such as by including pulse shaping circuitry within, or in addition to, the threshold detector 16), it is preferable to derive the output signal differently in order to minimize the risk of providing an improper "arm" and "fire" type output signal, while at the same time ensuring that the output signal remains free of multiple, unwanted transitions.